We are currently studying the role of lysosomes in corpus luteum function. During the period when luteal cells are developing and actively secreting progesterone, the levels and latency of several acid hydrolases are measured in the ovaries of superovulated rats. The effects of lysosomotropic agents on steroid secretion are studied during incubation of dispersed luteal cells and rat granulosa cells growing in tissue culture. In the culture system, mechanisms whereby luteal cells process lipoprotein bound cholesteryl esters are being investigated by following the metabolism of lipoproteins reconstituted with radiolabelled sterol esters or fluorescent derivatives of the esters. Current findings indicate that the lysosomes are involved in providing free cholesterol to luteal cells for progesterone formation, and that during the period of maximum steroidogenesis the lysosomes accumulated lipids, thus changing their buoyant density during gradient centrifugation. Hence it would appear that lysosomes represent a site for intracellular regulation of corpus luteum function.